Since the discovery of electroluminescent lamps (EL lamps) in 1939 by Destriau in Paris, engineers and scientists have held out the hope that these new lamps would be a commercially useful alternative to conventional incandescent and fluorescent lamps. To date, however, these promises have remained mainly unfulfilled due to a variety of technical and scientific problems that have continually hindered the progress of EL lamp design. This erratic progress has not been for a lack of scientific and engineering endeavor, that has been spurred by the fact that the commercial applications for bright, cold, flexible EL lamps which are inexpensive to produce and which consume little power are immense and the financial rewards correspondingly large.
Most recently, engineers and scientists involved in EL lamp design have nearly universally concentrated their efforts in a laminar EL lamp design based on indium tin oxide (ITO) sputtered onto a polyethylene terephthalate (PET) film substrate. In this design, a phosphor material is typically sandwiched between a back electrode often made of a metal foil such as aluminum foil, and a translucent sputtered ITO coated PET film front electrode. These lamps are operated by applying an A.C. voltage across the electrodes and this causes the phosphor material to luminesce. In order for the light produced in the phosphor material to escape from the lamp, one or more of the electrodes must be translucent or transparent. This translucent electrode was originally made of conducting glass paper or a conducting transparent plastic; currently, however, nearly all EL lamps use a sputtered ITO coated PET film as the translucent electrode.
With the discovery of ITO, EL lamp designers believed they finally had a material that would allow the early hopes for EL lamps to be fulfilled. Unfortunately, this again turned out to be a somewhat false hope. Although the sputtered ITO based EL lamps currently produced are brighter and more durable than the original lamps of 50 years ago, these lamps still possess a number of drawbacks that have precluded their use in many everyday applications.
The main drawbacks of sputtered ITO based lamps are cost, processability, and electrical and optical characteristics. Although sputtered ITO coated PET films are easily available, they are expensive and can account for the bulk of the cost of materials for EL lamps. In addition to this excessive cost, EL lamps based on sputtered ITO coated PET films are not easily shaped. When using sputtered ITO coated films in a typical lamp construction, the ITO layer will crack when the EL lamp is bent creased or stretched. Sputtered ITO/PET based lamps cannot therefore easily be shaped using well known polymer processing techniques such as sheet forming techniques. For these reasons the sputtered ITO based lamps cannot easily be shaped into anything other than the simplest geometric forms. One further drawback of lamps based on sputtered ITO coated PET films is that their brightness and spectral properties (i.e., wavelength of light emitted) is severely limited by the range of frequencies and voltages that may be used to drive the lamps. In a typical lamp construction, sputtered ITO is deposited as an extremely thin layer and consequently the lamps cannot carry much current and cannot be driven at high frequencies. To ensure that the conventional lamps have a long enough lifetime to be commercially useful, it is necessary to drive sputtered ITO based lamps at relatively low frequency and voltage and this severely restricts the brightness of the resulting lamp. One further drawback of sputtered ITO based lamps is the lack of manufacturing reliability. The exacting processing requirements for manufacture of conventional sputtered ITO based lamps result in low yields. Typically only 60-75% of manufactured lamps pass quality control and are commercially useful.
As described above, the majority of EL lamps are operated by applying a voltage across front and back electrodes separated by the light emitting layer; however, there is an alternative design in which one of the electrodes is split into two and the voltage is applied across the two halves of the split electrode. In this "split-electrode" design, the EL lamp still incorporates the other "unsplit" electrode that in most cases is not connected to the voltage source and functions as what is called a "floating" electrode. Examples of split-electrode lamp construction are disclosed in U.S. Pat. Nos. 2,928,974, 4,534,734, and 5,045,755. It is important to note that in these split electrode lamps, the floating electrode is an integral part of the lamp construction and the lamp would not function if the floating electrode were removed. In operation, power is supplied to the two split electrodes which capacitively couple to the floating electrode and this causes a voltage difference between the split electrodes and the floating electrode over the entire area of the electrodes. Light is therefore emitted from the entire volume of light emitting material sandwiched between the split electrodes and the floating electrodes. As in the case of the unsplit electrode lamps, one of the electrodes must be transparent or translucent to allow light to be emitted from the lamp. The split electrode lamps operate in fundamentally the same manner as the standard unsplit lamps and they suffer from exactly the same cost, processability and optical characteristic drawbacks as the unsplit versions. In fact, since the split electrode lamps require extra or more involved production steps to produce the split electrodes, split-electrode lamps may be more difficult and therefore more costly to produce than the standard laminar lamps.
In summary then: The discovery and development of EL lamps held out the hope of inexpensive, flexible lamps that do not generate much heat and which consume very little power. Unfortunately, fifty years of development has failed to fulfill many of these initial promises. Present day lamps typically are of laminar construction based on a PET mounted transparent sputtered ITO electrode. These lamps suffer from a variety of drawbacks that make their commercialization for many applications unfeasible. Important among these drawbacks are cost, lack of easy processability, low manufacturing yields, and restricted lamp brightness.